<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>过渡</title>
    <style>
        .root{
            width: 100px;
            height: 100px;
            bac-color: #000;
            transition: 2s ;/*可省略 ease 0s*/
        }
        .root:hover{
            width: 400px;
        }
    </style>
</head>
<body>

</body>
</html>